[manbart]

Convenience. You can copy/paste without touching the keyboard at all

[bilkow]

You don't need primary selection to avoid the keyboard, you can also hold right-click on your selection and release it on "copy" (or right-click on your selection then left-click on copy, more intuitive but slightly more cumbersome)

I agree it's less convenient (there's an extra step: explicitly copying the text), but in my experience it's also more reliable as you don't lose it by just selecting anything.